The present invention relates to an image detection apparatus which captures a high resolution surface image of a sample made of various materials and having an uneven surface, such as a semiconductor wafer on which fine circuit patterns are formed, and a defect detection/measurement apparatus using that image. In particular, this invention is preferably applicable to an apparatus which employs a high resolution and narrow focal depth optical system such as an optical microscope using a DUV light source.
As systems for picking up a high resolution surface image of a sample made of various materials and having an uneven surface, there are pattern defect detection apparatuses for semiconductor wafers.
In Japanese Patent Laid-Open No. 4-76450, a focus detection method is disclosed for semiconductor wafer pattern defect detection apparatuses. In this conventional method, a reticule inserted in an illuminating light path is projected onto the sample via an objective lens and an image of the reticule pattern reflected from the sample is shaded by another reticule placed in the detection light path. The height of the sample is detected by the increase or decrease in the quantity of light which has passed the reticule.
Similar to pattern defect detection apparatuses, projection exposure apparatuses also have a high resolution optical system which must be focused on the surface of a sample made of various materials and having an uneven surface. For automatic focusing in a projection exposure apparatus, a height detection method is disclosed in Japanese Patent Laid-Open No. 9-36036. Wafers to be processed by a projection exposure apparatus is coated entirely with a resist, an optically transparent-substance. To accurately detect the surface height of the resist in this disclosed method, S-polarized light is irradiated to the wafer at an incident angle of 85 degrees or more so that reflection at the transparent surface layer is raised.
The first conventional technique mentioned above is immune to the unevenness of the surface of the sample and the distribution of reflection over the surface of the sample. However, it has a drawback that the detected height does not necessarily agrees with that of the surface layer since where the surface layer is an optically transparent substance, irradiated light penetrates through the surface layer and is reflected by an under layer.
The second conventional technique mentioned above provides high accuracy height detection if the whole surface of the wafer is smoothly coated with a resist. However, sufficient care is not given to steep dents and bumps which may occur to the circuit pattern on the wafer after a resist pattern is formed or removed.